1. Field of the Invention
Polymeric substances which have the ability to absorb large amounts of aqueous fluids are well known in the prior art and are typically referred to as "superabsorbents." Starch-based superabsorbents are particularly useful, and are easily prepared by first graft polymerizing either acrylonitrile or mixtures of monomers that contain predominantly acrylonitrile onto either starch or flour and then, in a second step, saponifying the polyacrylonitrile moiety by treating the graft copolymer with hot alkali. This process has been reviewed by Fanta and Bagley in the Encyclopedia of Polymer Science and Technology, Supplement Vol. 2 (H. F. Mark and N. M. Bikales, eds., John Wiley & Sons, 1977, p. 665). There are a multitude of uses for superabsorbents. For example, they are used in agriculture to increase the water-holding capacity of poor or marginal soils, as seed coatings to enhance germination, and as root dipping compositions to reduce or eliminate transplant shock. Superabsorbents find application in disposable soft goods, such as diapers and feminine napkins, for enhancing the absorbtivity of these articles toward body fluids. Another important use for these substances is as thickening agents for aqueous systems. Medical applications include incorporation into body powders and wound dressings, particularly for the treatment of decubitus ulcers or bed sores.
Although superabsorbents enjoy a broad fluid of application, their cost is often an inhibiting factor in their widespread commercial acceptance, particularly in agriculture. Diluting the absorbent with an inexpensive extender, such as starch, has obvious marketing advantages. However, it is at once apparent that addition of an inert diluent will decrease the water absorbency of the resulting blend in an amount proportional to the amount of diluent added. This invention relates to diluting superabsorbents without a proportionate sacrifice of absorbency.
2. Description of the Prior Art
In U.S. Pat. No. 3,935,099 and related U.S. Pat. Nos. 3,981,100, 3,985,616, and 3,997,484, Weaver et al. teach that absorbent polymers may be extended by mixing dispersions of the saponified, gelatinized starch-polyacrylonitrile graft copolymers (GS-HPAN) with dispersions of inexpensive natural polymers, or their derivatives, and then drying the resulting mixtures. Exemplary extenders include flour, guar, gelatin, starch and dextrin. In Example 20 and accompanying Table 9, Weaver et al. illustrate the expected diluent effect on absorbency of several of these extenders.